This invention relates generally to lenses and to photodetectors which may be used, for example, for digital imaging devices including cameras and scanners.
Photodetectors are used to detect light in a digital imaging device. A large number of photodetectors may each detect a light intensity value for a given pixel. These light intensity values are digitized and used to form a digital depiction of a particular image. Such imaging devices are used in digital cameras, scanners and other devices.
The imaging device may be formed, for example, using complementary metal oxide semiconductor (CMOS) technology. Imaging devices made using CMOS technology include active pixel sensors (APS) and passive pixel sensors (PPS). Alternatively, the imaging device may use charge coupled device (CCD) technology. All of these devices can be made of particularly small size.
Imaging arrays of particularly small size may have a fill factor problem. The fill factor is the fraction of the total amount of light that falls on any given pixel. Ideally, the amount of light that falls on each photodetector is designed to be as high as possible to improve the signal to noise ratio. In order to improve the fill factor with small photodetectors, a microlens may be situated over the photodetector. The microlens focuses all the light incident on a pixel onto the photodetector and therefore improves the fill factor of the photodetector.
Conventional lens systems used in imaging devices employ high quality refractive lenses which are more expensive. These lenses, with high numerical apertures, may be used to reduce chromatic aberrations and dispersion. Dispersion and chromatic aberrations result in focal shifts that affect different colors (i.e. different wavelengths of light) differently. Dispersion, may be measured and expressed as an Abbe or V number. Even with expensive refractive lenses focal length variations due to temperature and wavelength may degrade performance.
Therefore, it would be desirable to have a lens system with improved characteristics.